greeter:fixed some bug of user switch

1.when getting in greeter by Switch-User from dde-lock or dde-shutdown:
the menu of user icons will show,and if you choose by keyboard.
There will be a bug that the icon  you choose by Enter is not the icon
shown .

2.Keep the function that you can switch user by keyboard without
clicking the switch-user icon in the right-down corner.

3.delete some useless code of switch-user
    ---Switch user by the left and right Arrow near the user icon

tower:https://tower.im/projects/1c0cd0c59ef941298c6e6b2ba6833b91/todos/28d8b3373bbb49d09bf0b7e4a22f5538/

Change-Id: I2c5fbd127f540fb4624e73a752cd3c91bde31357